OSOcean ScienceOSOcean Sci.1812-0792Copernicus PublicationsGöttingen, Germany10.5194/os-14-731-2018Transport, properties, and life cycles of mesoscale eddies in the eastern
tropical South PacificTransport, properties, and life cycles of mesoscale eddiesCzeschelRenarczeschel@geomar.deSchütteFlorianWellerRobert A.StrammaLothar1GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker
Weg 20, 24105 Kiel, Germany2Woods Hole Oceanographic Institution (WHOI), 266 Woods Hole Rd, Woods
Hole, MA 02543, USARena Czeschel (rczeschel@geomar.de)31July201814473175015January201831January201814May20189July2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://www.ocean-sci.net/14/731/2018/os-14-731-2018.htmlThe full text article is available as a PDF file from https://www.ocean-sci.net/14/731/2018/os-14-731-2018.pdf

The influence of mesoscale eddies on the flow field and the water masses, especially the
oxygen distribution of the eastern tropical South Pacific, is investigated
from a mooring, float, and satellite data set. Two anticyclonic (ACE1/2), one
mode-water (MWE), and one cyclonic eddy (CE) are identified and followed in
detail with satellite data on their westward transition with velocities of
3.2 to 6.0 cm s-1 from their generation
region, the shelf of the Peruvian and Chilean upwelling regime, across the
Stratus Ocean Reference Station (ORS; ∼20∘ S, 85∘ W)
to their decaying region far west in the oligotrophic open ocean. The ORS is
located in the transition zone between the oxygen minimum zone and the well
oxygenated South Pacific subtropical gyre. Velocity, hydrographic, and oxygen
measurements at the mooring show the impact of eddies on the weak flow region
of the eastern tropical South Pacific. Strong anomalies are related to the
passage of eddies and are not associated with a seasonal signal in the open
ocean. The mass transport of the four observed eddies across 85∘ W
is between 1.1 and 1.8 Sv. The eddy type-dependent available heat, salt, and
oxygen anomalies are 8.1×1018 J (ACE2), 1.0×1018 J
(MWE), and -8.9×1018 J (CE) for heat; 25.2×1010 kg
(ACE2), -3.1×1010 kg (MWE), and -41.5×1010 kg (CE) for
salt; and -3.6×1016µmol (ACE2),
-3.5×1016µmol (MWE), and
-6.5×1016µmol (CE) for oxygen showing a strong imbalance
between anticyclones and cyclones for salt transports probably due to
seasonal variability in water mass properties in the formation region of the
eddies. Heat, salt, and oxygen fluxes out of the coastal region across the
ORS region in the oligotrophic open South Pacific are estimated based on
these eddy anomalies and on eddy statistics (gained out of 23 years of
satellite data). Furthermore, four profiling floats were trapped in the ACE2
during its westward propagation between the formation region and the open
ocean, which allows for conclusions on lateral mixing of water mass
properties with time between the core of the eddy and the surrounding water.
The strongest lateral mixing was found between the seasonal thermocline and
the eddy core during the first half of the eddy lifetime.

(a) Percentage of eddy coverage determined from Aviso sea
level anomaly (SLA) during the period from 1993 to 2015. The mean
distribution of (b) oxygen, (c) temperature, and (d) salinity on density surface
26.6 kg m-3 derived from the monthly isopycnal and mixed-layer ocean climatology (MIMOC; Schmidtko et al., 2013).
The black ×'s show the
location of the float deployments, the westernmost × with the circle is also
the location of the Stratus mooring. The green, red, and black lines
represent trajectories of the anticyclones (ACE1, ACE2) and the anticyclonic
mode-water eddy (MWE), whereas the blue line is associated with the trajectory
of a cyclone (CE). Dashed lines show the extrapolated tracks to the
formation regions and the estimated time of formation. All of these eddies
have crossed close to the mooring position and are examined in more detail.

Introduction

The eastern tropical South Pacific (ETSP) containing the Peruvian upwelling
regime, which is one of the four major eastern boundary upwelling systems,
shows pronounced mesoscale and sub-mesoscale variability (e.g. Capet et al.,
2008; McWilliams et al., 2009; Chaigneau et al., 2011). Mesoscale
variability in the ocean occurs as linear Rossby waves and as nonlinear
vortices or eddies. During the last two decades eddies have been recognized
to play an important role in the vertical and horizontal transport of
momentum, heat, mass, and chemical constituents of seawater (e.g. Chelton et
al., 2007; Klein and Lapeyre, 2009) and therefore contribute to the
large-scale water mass distribution. Especially in upwelling areas, eddies
have been identified as major agents for the exchange between coastal waters
and the open ocean (e.g. Chaigneau et al., 2008; Pegliasco et al., 2015;
Schütte et al., 2016a). At least three types of eddies have been
identified: cyclonic, anticyclonic, and anticyclonic mode-water eddies (e.g.
McWilliams, 1985; D'Asaro, 1988; McGillicuddy Jr. et al., 2007), as well as a
transition from cyclonic eddies to “cyclonic thinnies” to exist throughout
the world ocean (McGillicuddy Jr., 2015). Usually, isopycnals in anticyclonic
eddies are depressed for the entire vertical extent of the eddy, while in
mode-water eddies a thick lens of water deepens the main thermocline while
shoaling the seasonal thermocline (McGillicuddy Jr. et al., 2007). Mode-water eddies
are also often referred to as intrathermocline eddies (ITEs; Hormazabal et al., 2013).
Cyclones dome both the seasonal and main
pycnocline.

Several analyses of the mean eddy properties offshore of the Peruvian coast
have been conducted in the last decade and found the largest eddy frequency in the
ETSP off Chimbote (∼9∘ S) and south of San Juan
(15∘ S) (e.g. Chaigneau et al., 2008, or Fig. 1a). Using a
combination of Argo float profiles and satellite data, the three-dimensional
mean eddy structure of the eastern South Pacific was described for the
temperature, salinity, density, and geostrophic velocity field of cyclones
as well as anticyclones (Chaigneau et al., 2011). However, a distinction in
“regular” anticyclones and anticyclonic mode-water eddies is still pending
in the ETSP. From recent findings (Stramma et al., 2013; Schütte et al.,
2016a, b) it seems to be mandatory to distinguish
between these two eddy types as they strongly differ in their efficiency to
transport conservative tracers, especially in upwelling areas. In addition
it is observed that the different eddy types influence non-conservative
tracers, like dissolved oxygen, in different ways within their isolated eddy
cores. Especially cyclones and anticyclonic mode-water eddies have been
reported to create an isolated biosphere, which greatly differs from the
biosphere present in the surrounding areas (Altabet et al., 2012;
Löscher et al., 2015). In these eddy cores the oxygen concentration can
decrease with time (Fiedler et al., 2016; Schütte et al., 2016b). In
wide areas of the world ocean eddies with an open-ocean low-oxygen core are
observed (e.g. North Pacific: Lukas and Santiano-Mandujano, 2001; South
Pacific: Stramma et al., 2013; tropical North Atlantic: Karstensen et al.,
2015). These low-oxygen eddies have strong impacts on sensible metazoan
communities and marine life (Hauss et al., 2016). Anammox is the leading
nitrogen loss process in ETSP eddies whereas denitrification was
undetectable (Callbeck et al., 2017), while denitrification appears only
patchy in the ETSP (Dalsgaard et al., 2012). Low-oxygen eddies release a
strong negative oxygen anomaly during their decay, which may influence the
large-scale oxygen distribution (Schütte et al., 2016b).

The following paper includes an analysis of the three different eddy types
and their impact on the water masses and oxygen distribution in the ETSP and
is based on the Stratus Ocean Reference Station (ORS) mooring. The Stratus
mooring is located at ∼20∘ S, 85∘ W in
the transition zone between the oxygen minimum zone (OMZ) and the
well oxygenated subtropical gyre (e.g. Tsuchiya and Talley, 1998). Eddy
analyses were also done in the past at the Stratus mooring, where a snapshot
of a strong anticyclonic mode-water eddy was observed in March/April 2012
(Stramma et al., 2014). In this paper we investigate in more detail the
mooring period March 2014 to April 2015 and set a focus on the isolation and
development of an eddy core during its isolated lifetime. We intensively
followed eddies of each kind (two “regular” anticyclones, one anticyclonic
mode-water eddy and one cyclone) which crossed the Stratus mooring position,
from their formation areas near the coast to their decay eastwards of the
Stratus mooring (Fig. 1). During their lifetime one of these eddies was
also partly sampled by several profiling floats equipped with oxygen sensors
which were deployed in March 2014 within the eddies (Fig. 1).

In general, the large-scale oxygen distribution in the ETSP is dominated by a
strong OMZ at depths of 100–900 m with minimum oxygen values at about
350 m depth (σθ=26.8 kg m-3) and suboxic conditions of
<4.5µmol kg-1 off Peru (e.g. Karstensen et al.,
2008; Paulmier and Ruiz-Pino, 2009, or Fig. 1b). In the OMZ the oxycline of 60 µmol kg-1 extends along the South American coast from
35∘ S to the Equator where it reaches westward to nearly
160∘ E (Llanillo et al., 2018).

In the ETSP the zonal tropical current bands supply oxygen-rich
water (O2) to the OMZ (Stramma et al., 2010). In contrast, the mid-depth
circulation in the eastern South Pacific Ocean is sluggish in the region of
the OMZ. As the mean currents are weak, eddy variability strongly influences
the flow and ultimately supplies oxygen-poor water to the OMZ (Czeschel et
al., 2011). A rough estimate of the oxygen budget of the eastern tropical
Pacific Ocean (Stramma et al., 2010) was used to estimate 22 % by vertical
mixing, 33 % by advection, and the largest component of 45 % by eddy
mixing (Brandt et al., 2015).

The mean upper ocean circulation of the ETSP is relatively complex exhibiting
several surface and subsurface currents. It is described to be composed of
the South Pacific subtropical gyre with the north-eastern current band shown
to be located south of 10 to 15∘ S called the Humboldt Current,
South Equatorial Current, Oceanic Chile-Peru Current, or Peru Oceanic Current
(e.g. Kessler, 2006; Ayón et al., 2008), a set of several zonal current
bands between the subtropical gyre and the Equator (e.g. Kessler, 2006;
Czeschel et al., 2015), as well as poleward and equatorward current bands
near the South American continent (e.g. Chaigneau et al., 2013). The
shipboard zonal velocity component along about 86∘ W composed of
three acoustic Doppler current profiler (ADCP) surveys showed larger
regions with westward then eastward flow between 13 and 22∘ S
(Brandt et al., 2015), although influenced by eddy features in ADCP
measurements in November 2012 (Czeschel et al., 2015).

In general most of the eddies in the ETSP propagate westward originating
from eddy generation hotspots near the coast following different eddy
corridors (Fig. 1a). Coastal water properties are captured within the
eddy cores and transported on their way into the open ocean across several
oxygen, temperature, and salinity gradients (Fig. 1b, c, d). The coastal
water mass properties differ, due to the upwelling, which is strongest in
the austral winter months from a seasonal cycle. The upwelled water near the
coast identified as Equatorial Subsurface Water (ESSW; e.g. Thomsen et al.,
2016) is colder, fresher, and less oxygenated in austral winter than in
austral summer.

This paper describes the temperature, salinity, and oxygen anomaly of the
different eddy types in the ETSP and their efficiency to dissipate the
existing gradients. Of special interest is the eddy type-dependent isolation
of the eddy cores during different eddy life stages. Knowledge about the
initial eddy-core conditions near the generation areas, measurements during
the mid-age of the eddy due to Argo floats, and measurements of the Stratus
mooring at the end of the eddy lifetime allows us to investigate the fluxes
associated with the eddies and the lateral mixing from the eddy-core water
masses with its surrounding waters.

Data setsStratus mooring

Since October 2000 the Stratus mooring has been maintained at about
20∘ S, 85.5∘ W mainly to collect an accurate record of
surface meteorology and air–sea fluxes of heat, freshwater, and momentum
(Colbo and Weller, 2009). In addition velocity, pressure, temperature,
and conductivity sensors (for salinity computation) and 13 oxygen sensors were
added to the mooring within the water column during the deployment period 8 March 2014 to 25 April 2015 at 19∘37′ S, 84∘57′ W
(velocity sensors used in this paper were also added during the deployment
period 6 April 2011 to 29 May 2012 at 19∘41′ S, 85∘34′ W). The depth distribution of the different measuring devices for the
2014 to 2015 deployment period are given in Table S1 and Fig. S1 in the Supplement (depth distribution of the velocity sensors for the 2011 to 2012
deployment period are listed in Stramma et al., 2014). Annual mean velocity
profiles were computed for the upper 600 m for the two Stratus mooring
deployment periods 2011/2012 and 2014/2015 where oxygen measurements were
conducted. To avoid influences of seasonal signals, only the period 10
to 9 April of the following year was computed and only instruments were used
which recorded the velocity for the entire period. These mean velocity
profiles can be compared with the October 2000 to December 2004 mean
velocity components (Colbo and Weller, 2007).

From the 13 oxygen sensors added to the 2014/2015 mooring period (Supplement Table S1),
3 instruments recorded erroneous oxygen values, which could
not be corrected after the recovery. The remaining 10 oxygen sensors
consist of 8 Aanderaa oxygen sensors in SeaGuard instruments, which
were used with the manufacturers calibration (accuracy <8µmol kg-1 or 5 %) and 2 oxygen loggers, which received an additional
lab calibration. For the 15 MicroCats (pressure, temperature and salinity),
a data calibration is done against shipboard CTD data during the service
cruises (RV Ron Brown, RB 14-01, and RV Cabo de Hornos) and later by
comparison with the data overlap with the previous mooring and by returning
the instruments to Sea-Bird Scientific for laboratory calibration. The SeaGuard
conductivity sensors in 107 and 350 m depth have an offset of -0.13
and -0.18 psu, respectively.

Satellite data

Satellite-derived sea level anomaly (SLA) data provided by the Copernicus
Marine and Environment Monitoring Service (CMEMS) were used to identify and
track the different eddies passing the Stratus mooring and to document the
position of the floats within the eddies. The delayed-time “all-sat-merged”
reference data set of SLA is used which is mapped on an 0.25∘×0.25∘ Cartesian grid and has a temporal
resolution of 1 day. The time period January 1993 to December 2015 was chosen
for the SLA and the geostrophic velocity anomalies are also provided by
CMEMS.

For sea surface chlorophyll (Chl) the MODIS/Aqua Level 3 data product mapped
on a 4 km grid available at http://oceancolor.gsfc.nasa.gov (last access: 18 June 2017) provided by
NASA is used. The time period January 2013 to December 2015 with a daily
resolution is chosen. Note that the Chl data are cloud dependent as they are measured via infrared light.

Argo floats

Seven profiling Argo floats with Aanderaa oxygen sensors were deployed in
March 2014 at 19∘36′ S, 84∘58′ W; 19∘27′ S,
83∘01′ W; 19∘15′ S, 80∘30′ W; and 18∘58′ S, 76∘59′ W.
The deployment locations (Fig. 1a) were chosen to
be close to anticyclonic or cyclonic eddies determined from SLA figures. The
floats were deployed in pairs with drifting depth at 400 and 1000 dbar and
cycling intervals of 10 days, except for 18∘58′ S, 76∘59′ W only one float was deployed at 400 dbar drifting depth.
From those seven
Argo floats, four floats remained for a longer period within eddies which
later crossed the Stratus mooring and are therefore used in more detail for
our calculations in the paper (the four Argo floats are: 6900527, 6900529,
6900530, and 6900532). Typically a full calibration of the oxygen sensors on
the Argo floats is not available. The different manufactures of Argo float
oxygen sensors specify their measurement error at least better than 8 µmol kg-1 or 5 %. Additionally the Argo float profiles of
temperature, salinity, and oxygen are compared and calibrated against the
measurements of the Stratus mooring and against each other giving a relative
accuracy.

Methods

From the Stratus mooring time series (from 8 March 2014 to 25 April 2015) of
velocity, temperature, salinity, and oxygen, eddies of each type are
identified and followed back and forward in time with the help of satellite
data. The focus is set on one mode-water eddy (MWE), two anticyclonic eddies
(ACE1, ACE2), and one cyclonic eddy (CE) as they are also sampled by Argo
floats (including oxygen sensors).

Heat, salt, and oxygen anomaly at the Stratus mooring

Available heat, salt, and oxygen anomalies (AHA, ASA, and AOA) were calculated
as described in Chaigneau et al. (2011) and Stramma et al. (2014). At the
Stratus mooring, eddy core anomalies were estimated by the difference between
the mean of temperature, salinity, and oxygen within the eddy boundaries and
the background field estimated from the annual mean for the period 10 April
2014 to 9 April 2015. Eddy boundaries are determined for every depth by the
mean of the maximum absolute values of the 90 h low-pass filtered southward
and northward velocity. The mean westward propagation of the eddies
estimated from SLA measurements is used to convert the time axis to a space
axis leading to a mean radius. The vertical extent is defined as the depth
of the coherent structure of the eddy, which is the ratio between the swirl
velocity U and the propagation velocity c of the eddy. If U/c>1, the feature is nonlinear and maintains its coherent structure while
propagating westward (Chelton et al., 2011). The swirl velocity is derived
from the mean of the absolute values of the maximum 90 h low-pass filtered
southward and northward velocity. Error bars for the horizontal eddy
boundaries are computed using the mean of the maximum absolute values of the
hourly-mean southward and northward velocities. As a result the swirl velocity
increases and likewise the vertical extent of the eddies due to the ratio
between swirl velocity U and propagation velocity c. Nonetheless, the
deviations of the horizontal boundaries of the eddy are small. The
deviations of the radius are used to estimate the error for AHA, ASA, and AOA
from uncertainties in the size of the eddies.

At the time when the mooring was deployed, part of the MWE had already
passed the mooring. Assuming a symmetric eddy, the centre of the MWE passed
the mooring on 8 March 2014 and fully passed the mooring until end of March
2014. The measurements of the eastern part of the eddy during that time span
were mirrored to obtain the full coverage of the MWE.

Determining properties of the MWE, ACE1/2, and CE conducted from
satellite data

The eddy shape is identified by analysing streamlines of the SLA-derived
geostrophic flow around an eddy centre (high/low SLA). Often the eddy
boundary is defined as the streamline with the strongest swirl velocity (for
more information on such an eddy detection algorithm see Nencioli et
al., 2010). For comparison of our results with the results of Chaigneau
et al. (2011) we also use the boundary definition of the streamline with the
strongest swirl velocity. Note that the identified areas are irregularly
circular therefore the circle-equivalent area is used to estimate the eddy
radius. Due to the resolution of the SLA data, the eddy radius must be at
least 45 km to unambiguously state that the identified area is a coherent
mesoscale eddy and not an artificial signal. Clearly identified individual
eddies may have a smaller radius than 45 km to get tracked. Eddies are
tracked forward and backward in time following the approach described by
Schütte et al. (2016a). To estimate the percentage of eddy coverage in
the ETSP, eddies are identified and tracked between 1993 and 2015. In the
following it was counted how often a grid point (0.5∘×0.5∘) was covered by an eddy structure. For the identification of
eddy generation areas, every newly detected eddy closer than 600 km off the
coast is counted in 1∘×1∘ boxes. The sum of all these
boxes is taken to compute the seasonal cycle of eddy generation. The Argo
float profiles and the mooring time series are separated into data conducted
within cyclones, anticyclones, and the “surrounding area” which is not
associated with eddy-like structures also following the approach of
Schütte et al. (2016a). In addition the relative position of the mooring
or Argo float profile in relation to the eddy centre and eddy boundary could
be computed.

Furthermore, the composites of the eddy surface signatures (SLA, SST, and
Chl) consist of 150×150 km snapshots around the identified eddy centres.
To exclude large-scale variations, the SST data are low-pass filtered
(cut-off wavelength of 15∘ longitude and 5∘ latitude)
and subtracted from the original data to preserve only the mesoscale
variability (see Schütte et al., 2016a, for more details).

ResultsGeneral eddy generation and its seasonal cycle in the ETSP

In the ETSP, 5244 eddies (49 % cyclones; 51 % anticyclones) are found
between January 1993 and December 2015 (requirement: having a radius between
45 and 150 km and visible for more than 7 days). Both types of eddies
have an average radius of about 70 km and on average 15 % of the ETSP are
covered everyday with eddies (Fig. 1a). Most of the eddies are generated
close to the Peruvian or Chilean coast, where large horizontal/vertical
shears exist in an otherwise quiescent region. In almost entire
agreement with Chaigneau et al. (2008), hotspot locations of eddy
generation are near the coast around 10∘ S and between
16 to 22∘ S (Fig. 2a, b). The four eddies (MWE, CE,
ACE1, and ACE1) described in detail below originate from the latter region.
After their generation near the coast, the anticyclonic eddies tend to
propagate north-westward, whereas cyclonic vortices migrate south-westward
(e.g. Chaigneau et al., 2008) into the open ocean. The seasonal cycle of
eddy generation, based on all new eddy detections closer than 600 km off the
coast, peaks in March and has its minimum in September (Fig. 2c), whereas
cyclonic eddies exhibit a stronger amplitude. However, both anticyclonic as
well as cyclonic eddies have their seasonal peak of formation in austral
summer/autumn (February/March) and the lowest number at the end of austral
spring (September; Fig. 2d).

The full eddy generation mechanisms are complex, whereby boundary current
separation due to a sharp topographic bend is one important aspect of the
eddy formation (Molemaker et al., 2015; Thomsen et al., 2016). It is
suggested that anticyclones are generated due to instabilities of the Peru
Chile Undercurrent (PCUC), whereas cyclonic eddies are formed from
instabilities of the equatorward surface currents (Chaigneau et al., 2013).
In this context the strength of the PCUC is essential (Thomsen et al.,
2016). Observations as well as models show a weak seasonal variability in
the PCUC off Peru which is stronger in austral summer and autumn (Thomsen et
al., 2016; Chaigneau et al., 2013; Penven et al., 2005) and might explain
the higher number of eddy generation during this season. Other model
simulations have revealed a seasonal cycle in eddy flux that peaks in
austral winter at the northern boundary of the OMZ, while it peaks a season
later at the southern boundary (Vergara et al., 2016). The PCUC also
experiences relatively strong fluctuations with periods of a few days to a
few weeks (Huyer et al., 1991).

Number of (a) anticyclones and (b) cyclones generated in
1∘×1∘ boxes (colours) between 1993 and 2015 closer than
600 km off the coast (coastal region). Seasonal cycle of the number of all
eddies (black line), anticyclones (red line), and cyclones (blue line)
generated in the coastal region are shown in (c) and (d).

Time series for the deployment period 8 March 2014 to 25 April 2015
at the position of the Stratus mooring (19∘37′ S,
84∘57′ W) for (a) weekly-delayed, high-pass filtered sea
level anomaly (in cm; blue curve) and geopotential anomaly between 450 and
295 m depth in m2 s-2 (orange curve), (c) oxygen
in µmol kg-1, (d) salinity, and (e) the
meridional velocity component in cm s-1, Hovmöller diagram
(time–latitude) at the longitude position of the Stratus mooring for (b) SLA
in cm. The white curve in (e) is the mixed layer depth defined for the depth
where the potential density anomaly is 0.125 kg m-3 larger than at the
surface. The black dots on the vertical line at the left mark the depths of
the used oxygen (c), conductivity (d), and velocity (e) sensors and the black
contour lines are selected density contours. Black solid (dashed) lines show
the date of the passages of the anticyclonic (cyclonic) eddies.

Eddy observations from March 2014 to April 2015 at the Stratus
mooring

From March 2014 to April 2015 the Stratus mooring was located at 19∘37′ S, 84∘57′ W, about 1500 km offshore in the
oligotrophic open ocean. Oxygen, salinity, and meridional velocity component
time series for the upper 600 m (Fig. 3; Supplement Fig. S2) record the
passage of several eddies between March 2014 and April 2015. These
observations are in agreement with the satellite data (SLA, SST, and Chl) at
the mooring location and the 450 to 295 m geopotential anomaly (Fig. 3a).

At the time of the mooring deployment on 8 March 2014 an anticyclonic MWE
with a radius of 43 km passed westward with the eddy centre to
the north of the mooring while a cyclonic eddy was located south of the
mooring site (Fig. 3b; Supplement movie M1). The mooring instruments
recorded the parameter distribution at the southern rim of the MWE revealing
anomalous low oxygen of less than 10 µmol kg-1 and anomalous
high salinity (temperature) of more than 34.65 psu (10.6 ∘C) in
the eddy core in 300 m depth. It was accompanied with an upward bending of
isopycnals above (∼250 m depth) and downward bending beneath
(∼350 m depth) the eddy core (Fig. 3c, d), which is typical
for a mode-water eddy in contrast to anticyclonic and cyclonic eddies. In
late March 2014 the MWE had passed the mooring.

A strong oxygen decrease as well as a salinity increase with a strong
downward displacement of the isopycnals at mid-depth in early August 2014
was related to an anticyclonic eddy (eddy ACE1; Fig. 3c) that passed the
mooring south of it (Fig. 3b). In the upper 250 m the oxygen concentration
increased with the maximum about 10 days later than the oxygen minimum at
290 to 600 m depth (Supplement Fig. S2). At 183 m depth, maxima in oxygen,
temperature and salinity of 265 µmol kg-1, 17.73 ∘C,
and 35.38 psu, respectively, were reached in mid-August 2014 reflecting the deepening of
the pycnocline which brings warmer, more saline and oxygen-rich waters to
deeper levels. The ACE1 shows meridional velocities of more than 5 cm s-1 in the upper 300 m depth (Fig. 3e).

Another strong oxygen decrease influenced the oxygen distribution from early
November 2014 to early January 2015. This anticyclonic eddy (eddy ACE2) had
a radius of 53 km and showed the strongest downward displacement of the
isopycnals in the 300 to 600 m range. At 350 m depth lowest oxygen values of
3.8 µmol kg-1 were reached in early December 2014 (Fig. 3c). The
massive deepening of the pycnoclines is also reflected by maxima in salinity
(34.69 psu) and temperature (10.33 ∘C) in 350 m depth (not shown).
The ACE2 shows high meridional velocities of more than 5 cm s-1 in the
upper 450 m depth.

Composite of the MWE (a, e, i), ACE1 (b, f, j),
ACE2 (c, g, k), and CE (d, h, l) surface signatures for
SLA, SST anomaly, and Chl. The dashed black and white line is the eddy
boundary, defined as the streamline of strongest velocity. The grey line
in (a) and (d) is the position of the Stratus mooring
during the eddy passage. The locations of the floats crossing the eddies are
marked by coloured dots (float #6900527 – red, #6900529 – orange,
#6900530 – green, #6900532 – blue).

(a) Eddy radius, (b) averaged maximum rotation velocity,
(c) nonlinearity parameter (U/c),
(d) Rossby number (U/fr), (e) centred SLA, and
(f) centred SST anomaly against normalized eddy lifetime of the
ACE1 (green), ACE2 (red), CE (blue), and MWE (black). The coloured solid
lines mark the passage at the Stratus mooring of the corresponding eddies.
The residence times of the four floats trapped in the ACE2 are marked by red
dashed lines in (a).

The lowest SLA and geopotential anomaly of the mooring deployment period was
connected to a strong upward displacement of the isopycnals in
February/March 2015 (Fig. 3c). The doming of the pycnoclines from January to
March 2015 is associated with the typical signature of cyclonic eddies,
which uplift colder, less saline and low-oxygen waters to shallower depths.
Low values for oxygen, temperature, and salinity at 183 m depth
of 69.1µmol kg-1, 11.7 ∘C and 34.54 psu in early February
2015 were related to a cyclonic eddy (CE) with a radius of 71 km. Water
properties of the CE may be associated with the Eastern South Pacific
Intermediate Water which is transported by equatorward surface currents
(Chaigneau et al., 2011).

According to the SLA satellite maps the centre of the MWE passed north of
the mooring (Fig. 3b). The centre of the westward propagating ACE1 and the
ACE2 passed the Stratus mooring only 14 km and 17 km, respectively, south of it,
hence the Stratus measurements were close to the centre of these two eddies
(Fig. 3b; Supplement movie M1). However, satellite data show that the
mooring captured only the northern segment of the ACE1 (Fig. 3b), therefore
the radius of 28 km determined from measurements at the Stratus mooring is
small in comparison to a mean radius of 40 km from satellite maps (Fig. 5a).
Oxygen anomalies from January to March 2015 are related by two
consecutive cyclonic eddies explaining the long-lasting and strong anomaly.
The first eddy (CE) passed the mooring 43 km north of it and then merged
with a second cyclonic eddy and passed to the south of the mooring (Fig. 3b).

Therefore, eddy events lead to a strong signal in water mass properties up
to 196 (40) µmol kg-1 in oxygen, 0.84 (0.18) psu in salinity, and
6.0 (1.9) ∘C in temperature in 183 m (350 m) depth. The oxygen
time series at 107 m depth (Supplement Fig. S2) does not show larger
anomalies from the mean values in March and late August, at the time the
Stratus mooring near sea surface temperature signal showed the maximum and
minimum of a seasonal signal (Colbo and Weller, 2007; their Fig. 3). Hence,
the maxima and minima described above for the Stratus time series at 183 and 350 m are clearly related to eddies and not related to the seasonal
signal in the upper layer of the open ocean.

(a) Swirl velocity vs. depth (solid lines), propagation
velocity (dashed lines), and vertical extent of the trapped fluid (circles)
of three anticyclonic eddies (MWE: grey, ACE1: green, ACE2: red) and a
cyclonic eddy (CE: blue). Profiles of anomalies of (b) temperature, (c)
salinity, and (d) oxygen (µmol kg-1) calculated as the difference
between the core of MWE (grey), ACE1 (green), ACE2 (red), and CE (blue) as well as the 1-year mean
of the Stratus mooring (10 April 2014–9 April 2015).

Net transport of heat, salt, and oxygen via eddies in the ETSP

Horizontal eddy transport can be explained by two mechanisms: (1) by eddy
stirring, which occurs at the periphery of the eddy (e.g. Gaube et al.,
2015; Chelton et al., 2011) and (2) by eddy transport of water masses trapped
in the eddy interior (Gaube, 2013, 2015). We are focusing on the latter
mechanism. The question of how much anomalous water properties an eddy is
able to trap and transport into the open ocean depends on the relation
between swirl velocity and propagation velocity. The MWE and the ACE2 have a
similar propagation velocity of 4.3 and 4.2 cm s-1, respectively. The
CE propagates fastest (6 cm s-1) and the ACE1 propagates slowest (3.2 cm s-1) (Fig. 6a),
which fits well to the mean westward propagation
speed of 3–6 and 4.3 cm s-1 estimated for eddies in the
region off Peru (Chaigneau et al., 2008, 2011).

The observed swirl velocity at the Stratus mooring is in accordance with other
values measured in the ETSP (Chaigneau et al., 2011; Stramma et al., 2013,
2014). All anticyclones (MWE, ACE1, and ACE2) show high rotation values in
the upper 200 m depth with maximum velocities of 11, 13, and
17 cm s-1, respectively (Fig. 6a), which is between the values of mean anticyclonic
eddies (9 cm s-1; Chaigneau et al., 2011) and a strong relatively young
anticyclonic coastal mode-water eddy (35 cm s-1; Stramma et al., 2013).
Further, the stronger swirl velocity of ACE2 agrees very well with an
anticyclonic eddy, which passed the Stratus mooring during September to
December 2011 at about the same season as for ACE2 (19 cm s-1; Stramma
et al., 2014). Typically for anticyclones, largest velocities occur in the
upper 250 m depth, whereas the MWE shows weak rotation in the near surface
and a deeper core instead. Below 250 m depth, the swirl velocity of ACE1 is
significantly weaker than the swirl velocity of ACE2. Nonetheless, due to
the much slower propagation velocity of ACE1 the fluid stays trapped within
the eddy (U/c>1) leading to a deep vertical extent of both
anticyclones of 504 m (ACE1) and 523 m (ACE2) as described for mean
anticyclones in the ETSP (Chaigneau et al., 2011).

Within the eddy boundaries of the two anticyclones (ACE1 and ACE2), positive
anomalies of temperature and salinity were observed between 100 and 600 m depth (Fig. 6b, c). Between 150 and 200 m depth maximum anomalies of
2.1 ∘C (ACE1) and 1.5 ∘C (ACE2) in temperature and 0.35 psu (ACE1) and 0.24 psu (ACE2) in salinity were measured which are
significantly higher than described for the mean of a composite of
anticyclones (0.8 ∘C, 0.08 psu; Chaigneau et al., 2011). Due to
the uplift (depression) of isopycnals above (below) 200 m depth the MWE
shows negative (positive) anomalies in temperature and salinity between 50
and 200 m (below 200 m) depth, which are weak in comparison to the other two
anticyclones.

Oxygen shows a mainly negative anomaly below 100, 220, and 280 m depth
respectively within the anticyclones MWE, AC1, and AC2. Both the MWE and the
ACE2 are having their largest negative anomalies in 250 m depth and a second
minimum in 450 m depth, which is just above and below the core of the OMZ
indicating the transport of low oxygenated water masses from a region with a
larger vertical expansion of the OMZ (Fig. 6d). In the core depth of the OMZ
at 350 m depth, only weak negative oxygen anomalies are possible as the
oxygen content is already low, but still the passage of the stronger
anticyclone ACE2 results in an oxygen decrease by 14 µmol kg-1.

Water mass anomalies within the MWE lead to available heat, salt, and
oxygen anomalies (AHA, ASA, AOA) of 1.0×1018 J, -3.1×1010 kg, and
-3.5×1016µmol. These values are about 5 times smaller in
comparison to a mode-water eddy that was also measured at the Stratus
mooring in February/March 2012 (Stramma et al., 2014; Table 1). ASA is even
negative in 2015 due to the strong doming in the upper 200 m. As both
mode-water eddies have about the same propagation speed and volume, the
differences in water mass properties point towards seasonal or interannual
variations of the water mass characteristics during their formation. The MWE
is generated in February, when upwelling-favourable alongshore winds weaken
and SST increases (Gutiérrez et al., 2011), whereas the mode-water eddy
observed by Stramma et al. (2014) is generated in April, when the PCUC,
which transports oxygen-deficient ESSW (Hormazabal et al., 2013), has its
poleward maximum (Shaffer et al., 1999; Penven et al., 2005; Chaigneau et
al., 2013). In addition, the MWE passed to the north of the Stratus mooring
during its deployment, hence the method to define the fully MWE parameter
might lead to higher deviations to the real eddy parameters.

Properties and available heat, salt, and oxygen anomalies (AHA, ASA,
AOA) with error bars of one mode-water eddy (MWE), two anticyclones (AE1 and
AE2), and one cyclonic eddy (CE) measured at the Stratus mooring in 2014/2015
within the vertical layer of the coherent structure in comparison to
measurements in February/March 2012 (Stramma et al., 2014; STR14, mode-water eddy)
at the Stratus mooring, at 16∘45′ S, 83∘50′ W in
November 2012 (Stramma et al., 2013; STR13, anticyclone) and mean values for
10–20∘ S relative to a mean climatology
(Chaigneau et al., 2011; CH11, anticyclones and cyclones). Based on instruments available, the
vertical extent for heat and salt computations (TS) and oxygen (OX) differs.
It is important to note that the radius of the ACE1 might be underestimated.

The volume of ACE2 (4.6×1012 m3) is in agreement with the mean
anticyclones (Chaigneau et al., 2011) and the open-ocean anticyclone (Stramma
et al., 2013) but 3 times larger than the weaker ACE1, which is partly
due to the underestimated radius (Table 1). The AHA, ASA, and AOA of ACE2 are
8.1×1018 J, 25.2×1010 kg, and -3.6×1016µmol and therefore far greater than the AHA, ASA, and
AOA of ACE1 (1.8×1018 J, 5.5×1010 kg, -0.02×1016µmol). The weak negative AOA of ACE1 result from the
strong and positive oxygen anomaly in the upper 280 m depth in comparison to
the background water mass. Strong differences between the results for ACE1
and ACE2 are of course due to the higher volume of the ACE2 but might also
reflect the conditions at different seasons during the formation of the
eddies leading to varying water mass properties. ACE1 (ACE2) is generated in
austral summer (winter) when upwelling-favourable winds weaken (strengthen).
Estimations of AHA and ASA within ACE2 match the mean values of Chaigneau et
al. (2011). In comparison to the open-ocean eddy (Stramma et al., 2013) the
AHA of the ACE2 is twice as high, but the AOA is only half as much (Table 1).

The cyclonic eddy in March 2015 (CE) has maximum velocities of
14 cm s-1 in 50 m depth (Fig. 6a). Due to the high translation speed
of the CE, the vertical extent of 176 m depth is much shallower than the
vertical extent of the anticyclones, which is consistent with the mean
cyclones (Chaigneau et al., 2011). The mean temperature shows a pronounced
negative anomaly within the eddy boundaries between 70 and 430 m depth with
a maximum anomaly of -2.3∘C in 160 m (Fig. 6b) resulting in a
negative AHA of -8.9×1018 J. The salinity is negative between
40 and 220 m depth with a maximum anomaly of -0.32 psu in 160 m depth
(Fig. 6c). This results in an extremely large negative ASA of -41.5×1010 kg. Oxygen shows a strong negative anomaly in the upper 320 m
having a maximum anomaly of -69µmol kg-1 in 180 m depth
(Fig. 6d) due to the uplift of the main thermocline leading to a negative AOA
of -6.5×1016µmol for the 107 to 176 m depth layer.
Although the volume of the CE is in good agreement with the mean values of
Chaigneau et al. (2011), the estimated AHA and ASA are much higher (Table 1),
which is likely due to strong seasonal variations during the generation of
the CE. The CE is formed in austral winter off Peru when coastal alongshore
winds intensify leading to an enhanced upwelling of cold and
nutrient-rich and oxygen-poor water due to high biological production.
Additionally, equatorward surface currents, which transport the relatively
cold and fresh Eastern South Pacific Intermediate Water, are strongest during
austral winter (Gunther, 1936).

Fluxes of mass, heat, salinity, and oxygen are estimated from the volume,
AHA, ASA, and AOA (Table 1) for the period in which the MWE, ACE2, and CE
cross the 85∘ W longitude at the Stratus mooring. The ratio of the
heat fluxes of the three different eddies mirror the differences between
volume, AHA, ASA, and AOA of the respective eddies because of the similar
duration of the passages of the eddies. Transports of mass anomaly for the
CE, ACE2, and MWE are again similar and range between 1.1 Sv for the CE and
1.8 Sv for ACE2 (Table 2). Due to the local conditions and different water
masses during its formation, the CE shows a strong negative transport of
heat (-3.8×1012 W) and salt (-17.5×104 kg s-1) across the
mooring, whereas the ACE2 transports the highest positive amount of heat
(3.2×1012 W) and salt (10.0×104 kg s-1) per year. Whereas
the transport of heat and salt of the MWE is relatively small in comparison
to the CE and ACE2, the transport of low oxygen water of -1.8×1010µmol s-1 is in the same range as the CE and ACE2 due to a thick
lens of low-oxygen water within the MWE.

Available anomalies of heat, salt, and oxygen of cyclonic and anticyclonic
eddies gained from the Stratus mooring and from the literature (Table 1) are
now used to estimate the relative contribution of long-lived eddies to fluxes
of mass, heat, salt, and oxygen in an offshore area of the ETSP. The mean
heat (in W), salt (in kg s-1), and oxygen transport
(µmol s-1) are calculated by multiplying the amount of AHA,
ASA, and AOA of the composite eddies with the number of eddies dissipating
per year in an offshore area (corresponding to a flux divergence). We define
an area over a north–south direction from 10 to 24∘ S. The
transition area is bordered in the east by a line running parallel to the
Peruvian and Chilean coast at a distance of 6∘ and in the west by the
90∘ W longitude corresponding to a size of ∼1.7×106 km2. Based on averaged satellite measurements, 58.6 eddies of
all eddies that are generated off the coast (Fig. 2) reach the offshore area
per year from which 28.9 are cyclones and 29.7 are anticyclones. Also, 2.1
cyclones and 0.7 anticyclones and mode-water eddies propagate into the area
west of the 90∘ W longitude meaning that 26.8 of the cyclones and 29
of the anticyclones and mode-water eddies have dissipated and therefore
transported a certain amount of heat, salt, and oxygen into the offshore
zone. Based on the mean of AHA, ASA, and AOA for the composite eddies, the
mean transport of heat (salt, oxygen) per year from the coastal region into
the transition zone is -6.4×1012 W (-2.4×105 kg s-1, -5.7×1010µmol kg-1 s-1) for cyclones and 4.7×1012 W (1.5×105 kg s-1, -5.9×1010µmol kg-1 s-1) for anticyclones and mode-water
eddies in agreement with estimates for transport anomalies of heat and salt
in this region by Chaigneau et al. (2011).

Heat and especially salt fluxes across the Stratus mooring as well as for
the ETSP reveal an imbalance between anticyclones and cyclones which are due
to a higher transport of anomalous cold and fresh water within cyclones from
the coast off Peru and Chile into the upper open ocean. Both types of eddies
show negative oxygen fluxes in the layer defined as the depth of the
coherent structure of the eddy (Table 1) meaning that anticyclones and
cyclones transport less oxygenated water into the upper and mid-depth open
ocean and therefore have an impact on the balance and size of the OMZ in the
ETSP, which is also confirmed by models (Frenger et al., 2018).

Properties of the observed eddies MWE, ACE1/2, and CE during their
lifetime

With the help of satellite data the four eddies (MWE, ACE1/2, and CE) could
be identified and followed from areas near the Peruvian and off the Chilean
coast to the areas of dissipation westwards of the Stratus mooring in the
open ocean. The trajectories of the three anticyclonic eddies (MWE, ACE1/2)
were extrapolated to the formation regions near the coast between
20 and 23∘ S (Fig. 1). The CE formed during end of
July 2014 off the Peruvian coast during the winter season when upwelling is
usually strong and decayed in mid-March 2015 after propagating 1200 km in
more than 7 months. Both the MWE and the ACE1 started off the Chilean
coast during the end of the summer season with usually low upwelling. The
MWE can be followed for about 2 years till March 2015 propagating 2880 km.
The ACE1 is tracked for 620 days until it decayed at the end of November 2014, after
propagating 1750 km. The ACE2, which was generated during the upwelling
season at the end of winter in September 2013 and decayed in June 2015, propagated
westward for 2350 km in 650 days.

As expected from the polarity depending meridional deflection of all eddies
(anticyclones – equatorward, cyclones – poleward), the individual
pathways of the ACE1 and ACE2 also show a north-westward direction whereas the CE
migrates more south-westwards (Fig. 1a). Note that the MWE shows no clear
meridional deflection on the way to the west.

Anticyclonic eddies (MWE, ACE1/2) are associated with a positive SLA, wherein
ACE2 shows the strongest mean elevation of all anticyclonic eddies of 8 cm
(Fig. 4c) and cyclonic eddies are identified by a negative SLA, wherein the
CE shows a mean minimum SLA of -2 cm in the centre of the eddy (Fig. 4d).
Nevertheless, the CE showed the largest SLA differences at the Stratus
mooring (Fig. 3a). In general, mode-water eddies are difficult to detect by
satellite altimetry due to a relatively weak velocity near the
surface (Fig. 6a), which is
generated by the typical distribution of the isopycnals. Therefore, it is
noteworthy that the MWE has a stronger SLA (7 cm) than the relatively weak
ACE1 (6 cm). The SLA of the MWE indicates higher variability during its
lifespan than the other eddies (Fig. 5e). The maximum SLA of the
anticyclones is
obtained during their mid-age, whereas the SLA of the CE decreases after the
very beginning.

Due to the uplift of the seasonal pycnocline in both eddies, MWE and
CE, cold and nutrient rich water is upwelled into the euphotic zone leading
to enhanced biological production, which is reflected by negative SST
anomalies of -0.04∘C (MWE, Fig. 4e), -0.08∘C (CE,
Fig. 4h), and a high chlorophyll concentration of 0.19 mg m-3 (CE, Fig. 4l).
Surprisingly, the mean SST (Chl) of the ACE1 and ACE2 are negative
(high) and around zero, respectively, as one would expect positive (low) SST
(Chl) anomalies due to the depression of the thermoclines. The development
of the SST predominantly shows negative anomalies with short periods of
positive anomalies for the anticyclones (Fig. 5f).

The development of further eddy properties (radius, km; rotating velocity,
m s-1; nonlinearity parameter; and Rossby number) during the
normalized lifespan are shown in Fig. 5 indicating that the observed
eddies pass the Stratus mooring during their mid-age (MWE and ACE2) and
during the end of their lifetime (ACE1 and CE). The anticyclones ACE1/2
have their maximum radius during the last third of their lifetime, whereas
the development of the radius of the MWE is symmetric and the radius of the
CE increases during the first third of its lifetime (Fig. 5a). Note the
decreasing maximum rotation velocity of the ACE2 during the second half of
its lifetime (Fig. 5b).

Water mass anomalies can only be preserved within an eddy if the feature is
nonlinear and maintains its coherent structure. During their full
lifetime, the nonlinear parameter U/c>1 for all eddies and
confirms the coherent feature (Fig. 5c).
Nonetheless, significant variations in the
nonlinear parameter U/c determined at the surface might indicate changes in
the volume of the eddies, which can be influenced by friction,
stratification, fluctuations of the mean flow, or the collapse with other
eddies. Maps of SLA show a permanent change of the radius due to an
irregular and varying shape and the merging with other eddies (Supplement:
movie M1), which makes it sometimes difficult to track an eddy during its
whole lifetime. Fluctuations are also produced by the coarse resolution of
the satellite data (0.25∘×0.25∘) and the merging algorithms used by AVISO.
However, the MWE and CE show stronger fluctuations of the nonlinear parameter than the ACE1/2,
which probably mirrors the higher variability in the swirl velocity of both
eddy types (Fig. 5b). Nonetheless, the nonlinear parameter U/c is always
higher than 1 and therefore indicates a trapped volume despite strong
fluctuations at the surface. The small variations in the eddy properties of
the ACE2 (Fig. 5a–e) suggest a relatively stable structure.

Distribution of oxygen (in µmol kg-1,
coloured) and density (black lines) vs. time in the upper 600 m depth of
floats 6900532 (a), 6900530 (b), 6900529 (c), and 6900527 (d) which have been
trapped within the ACE2 at different stages. The residence time in ACE2 of
the floats in April/May 2014, September 2014, October 2014, and December
2014/January 2015 is marked by dashed black lines. The white curve is the
mixed layer depth defined for the depth where the potential density anomaly
is 0.125 kg m-3 larger than at the surface.

All eddies indicate a Rossby number Ro<1 describing the
typical scale for mesoscale eddies (Fig. 5d). The mean life cycle of an eddy
consists of a growth and decaying phase, both lasting about 20 % of its
lifetime and a stable phase in between (Liu et al., 2012; Frenger et al.,
2015; Samelson et al., 2014). This is consistent with our observations
showing constant Rossby numbers of less than 0.1 between 0.2 and 0.8
reflecting a stable, geostrophic phase over 60 % of the lifetime of
all eddy types. At the beginning and at the end of the eddy lifetime the
Rossby numbers are increasing and indicating the influence of possible
ageostrophic processes. But the increase in the Rossby number for both
anticyclones is not as striking as for the MWE and CE. However, a very
detailed and exact discussion about the evolution of the Rossby radius (and
also the radius, the average maximum rotation, and the nonlinearity) and
possible ageostrophic processes is not possible due to the coarse resolution
of the underlying SLA data.

Same as Fig. 7 but for salinity distribution.

Profiles of (a) oxygen vs. density and (b) temperature–salinity diagrams at
the Stratus mooring (19∘37′ S, 84∘57′ W) for a
1-year mean (10 April 2014 to 9 April 2015; black) and during the passage of
the ACE2 in November 2014 (grey line), from the estimated formation region
at 23∘ S, 71∘ W from the MIMOC climatology (dark blue)
and from data of four floats (69005-27, -29, -30, and -32) trapped in the ACE2
(for coloured lines see legend). The age of the ACE2 in days during the
respective measurement is indicated in the upper panel. The age of the ACE2
(in days) at the time of the respective float measurements is marked in (a).

Observations of Argo floats within the eddy-core of ACE2 during
its mid-age

ACE2 has been tracked via SLA from March 2014 on, passed the Stratus mooring
in November 2014 and decayed in June 2015. During this period, four floats
(Fig. 7) were captured within the ACE2 during its westward propagation at
different times providing information about different stages of the eddy. The
first float (#6900532) was trapped in the period from mid-April to mid-May
2014 at 19.9∘ S, 77.1∘ W (Supplement: movie M1). The eddy
shows the core at about isopycnal 26.4 kg m-3 (∼180 m depth)
with extremely low oxygen of less than 4 µmol kg-1 between 150
and 400 m depth (Fig. 7a) and enhanced salinity of more than 34.8 psu in
the upper 240 m depth (Fig. 8a). The warm, salty, and oxygen-depleted water
mass of the core of the ACE2 coincides with the water mass of the likely
formation region of the ACE2 obtained from the monthly isopycnal and mixed-layer ocean climatology (MIMOC; Schmidtko et al., 2013; Fig. 9a, b)
reflecting the characteristics of the oxygen-depleted ESSW. The ESSW is
carried poleward by the secondary southern subsurface countercurrent (Montes
et al., 2014), feeds the subsurface PCUC (Hormazabal et al., 2013), and is
then transported along the Peruvian and Chilean coast where anticyclonic
eddies are likely generated (Chaigneau et al., 2011).

In September 2014, about 4 months later and more than 6∘ further west
at 19.4∘ S, 83.4∘ W a second float
(#6900530) was trapped in the same eddy ACE2 (Supplement: movie M1). The
core was still located at isopycnal 26.4 kg m-3 (∼230 m depth)
showing minimum oxygen values of less than 4 µmol kg-1
(Fig. 7b) and maximum salinity of more than 34.8 psu (Fig. 8b) between 200
and 280 m depth. However, the vertical extent of anomalously high salinity and
anomalously low oxygen has decreased (Fig. 9a, b) which is likely due to
lateral mixing. Mixing mostly takes place above the core of the eddy between
the density layers σθ=25.7 and σθ=26.3 kg m-3 (Supplement Fig. S3) with largest changes in oxygen
(0.5 µmol kg-1 day-1), temperature (-0.007∘C day-1),
and salinity (-0.002 psu day-1) at about σθ=26.0 kg m-3. This density level corresponds to a depth
between 100 and 170 m, where a high swirl velocity exists within the ACE2
(Fig. 6a), which is essential to keep up the coherent structure.

Shortly after, in October 2014, a third float (#6900529) stayed in the
ACE2 at about 20.4∘ S, 82.8∘ W. The strongest water
property anomaly is now located at isopycnal 26.6 kg m-3
(∼310 m depth) showing minimum oxygen of less than 8 µmol kg-1 (Fig. 7c). The salinity anomaly transported within the eddy
has declined further to about 34.65 psu (Fig. 8c). The development of
the water mass properties within the eddy points towards mixing along
density surfaces between σθ=26.0 kg m-3
(∼190 m) and σθ=26.5 kg m-3 (280 m). The changes are strongest above the core of the eddy at about σθ=26.3 kg m-3 (∼205 to 240 m) showing
the mixing of oxygen-rich (2.8 µmol kg-1 day-1), colder
(-0.07∘C day-1) and fresher water (-0.017 psu day-1) into the ACE2 (Supplement Fig. S3).

Decreased anomalies might also be related to the fact that the float did not
capture the eddy centre as it was located in the south-east rim of the eddy
(Fig. 4c). Whereas the salinity measurements of the float differ from those
of the Stratus measurements obtained during the passage of the ACE2 (Fig. 9b), the oxygen anomaly transported in the core agrees well (Fig. 9a).

After the ACE2 has passed the Stratus mooring in November 2014, the last of
the four floats (#6900527) was trapped at the southern rim of the eddy
(Fig. 4c) from December 2014 to January 2015 at about 20.4∘ S,
86.4∘ W. The eddy core is still clearly visible, although water mass
properties within the core of the ACE2 has further changed (oxygen
>10µmol kg-1, Fig. 7d) and the vertical extent
of the eddy has declined. Mixing of slightly oxygen-richer water can be
observed over the whole water column (Supplement Fig. S3a), whereas warmer
and more saline water is entrained in the upper part of the eddy (σθ>26.3 kg m-3≅240 m depth) and colder and
fresher water below. These changes might also be due to the location of the
float outside the eddy boundary.

Discussion and outlook

The ETSP is known for its high eddy frequency (Chaigneau et al., 2008).
There is still limited knowledge in this region about the dynamics of eddies
especially on their effective transport and their dissipation. In this study
the activity of three different types of eddies (mode water, anticyclonic,
and cyclonic eddy) during their westward propagation was investigated from
the formation area in the upwelling area off Peru and Chile into the open
ocean. The focus was on the development of the eddies, seasonal conditions
during their formation, their life cycles and fluxes, and the change of water
mass properties transported within the isolated eddies using a broad range
of observational data such as SLA, SST, and Chl from satellites as well as
hydrographic data and oxygen from the Stratus mooring and from Argo floats.

Available heat, salt, and oxygen anomalies could be computed for the
investigated eddies. Generally, heat and salt anomalies transported within
eddies are positive for anticyclones and negative for cyclones and might be
compensated for as they are of about the same amount (Chaigneau et al., 2011).
In this study the negative and positive heat anomalies of the CE
(-8.9×1018 J), ACE2 (8.1×1018 J), and MWE (1.0×1018 J)
almost balance each other. In contrast, the sum of negative and positive salt anomalies transported within the CE (-41.5×1010 kg),
ACE2 (25.2×1010 kg), and MWE (-3.1×1010 kg) is unbalanced. The
AOA is negative for all types of eddies (MWE: -3.5×1016µmol;
ACE2: -3.6×1016µmol; CE: -6.5×1016µmol), whereby
the transport of oxygen-poor water from the upwelling region into the open
ocean is more surface intensified due to the shallow structure of the
cyclonic eddies. A mode-water eddy observed by Stramma et al. (2014) was
generated in year 2011, which is considered as a La Niña period, when
generally lower oxygen and higher salinity values exist in the upper 100 m depth (Stramma et al., 2016) leading to higher anomalies of salt and oxygen
of the mode-water eddy observed by Stramma et al. (2014) in comparison to the actual
MWE. Therefore, seasonal variability such as fluctuation of alongshore
upwelling-favourable winds off Peru and Chile as well as interannual
variability such as El Niño or La Niña have an impact on the water mass
properties trapped and transported within eddies from the coast of Peru and
Chile into the open ocean reflecting the high variability of AHA and ASA.

For the Atlantic Ocean the low-oxygen eddy cores have been attributed to high
productivity in the surface (Schütte et al., 2016b), enhanced
respiration of sinking organic material at subsurface depth (Fiedler et al.,
2016), and a strong isolation of the eddy core (Karstensen et al., 2017). An
anticyclonic mode-water eddy observed in the Pacific at the Stratus mooring
in February/March 2012 indicated high primary production just below the
mixed layer (Stramma et al., 2014). According to a global investigation of
Argo floats, the eastern South Pacific off Peru and Chile seems to have the
highest amount of MWEs, which are also deep reaching compared to other
regions (Zhang et al., 2017; their Fig. 2). Nevertheless, in the mooring
deployment period 2014/2015 only one MWE crossed to the north of the mooring
and the results have to be regarded with caution. Even though the AHA and
ASA of the MWE are small in comparison to both the anticyclonic eddies and
the cyclonic eddy the transport of low oxygen water is in the same range as
the other eddies due to the typical thick lens of low oxygen water within
mode-water eddies.

From a combination of satellite data and Argo profiles, long-lived eddies
(lifetime longer than 30 days) in the Peru-Chile upwelling system (55 % of
the sampled anticyclonic eddies) had subsurface-intensified maximum
temperature and salinity anomalies below the seasonal pycnocline, whereas
88 % of the cyclonic eddies are surface intensified (Pegliasco et al.,
2015). The 55 % subsurface-intensified anticyclonic eddies represent mode-water eddies while the 45 % surface intensified anticyclones are
“regular”
anticyclonic eddies. Observations and model results for the California
Current system showed a good agreement between observed and modelled eddy
structures (Kurian et al., 2011).

Satellite-based estimate of the surface-layer eddy heat flux divergence,
while large in coastal regions, is small when averaged over the south-east
Pacific Ocean, suggesting that eddies do not substantially contribute to
cooling the surface layer in this region (Holte et al., 2013). In this study,
the release of fluxes of heat (cyclones: -6.4×1012 W;
anticyclones: 4.7×1012 W) and salt (-2.4×105 kg s-1; 1.5×105 kg s-1) estimated from
long-lived eddies dissipating in the ETSP confirms the discrepancy between
different types of eddies leading to a net transport of colder and fresher
water from the formation regions off Peru and Chile into the open ocean. In
contrast, all three types of eddies show a negative oxygen flux of -5.7×1011µmol kg-1 s-1 for cyclones and -5.9×1010µmol kg-1 s-1 for anticyclones and
mode-water eddies pointing towards an active role of eddies in balancing and
shaping the OMZ confirmed by model studies (Frenger et al., 2018).

Thus, the variability of eddy generation on different timescales might be an
important factor for the variability in the OMZ. Eddy generation off the
coast between 8 and 24∘ S peak in austral
summer/spring, which agrees with the strengthening of the PCUC and the
possible mechanism of the generation of eddies due to instabilities.
However, this is not in agreement with model simulations showing an
eddy-induced offshore transport off Peru that peaks in austral spring
(winter) at the southern (northern) boundary of the OMZ (Vergara et al.,
2016). As the PCUC also shows strong fluctuations lasting a few days up to a
few weeks (Huyer et al., 1991) it might be difficult to determine a seasonal
dimension between the PCUC and the rate of eddy formation. El Niño (La
Niña) events deepen (shoal) the thermocline and intensify (weaken) the
PCUC (e.g. Montes et al., 2011; Combes et al., 2015). Intraseasonal and
interannual variability is another factor modulating the strength of the
PCUC off Chile and therefore the formation rate of anticyclonic eddies
(Shaffer et al., 1999).

In this study the development of isolated water mass properties was
investigated on the basis of four floats trapped within an anticyclonic eddy
indicating lateral mixing of all water properties (oxygen, temperature, and
salinity). The water mass within the core of the ACE2 shows the typical
characteristics of the salty and oxygen-depleted ESSW. The mixing is
strongest between the seasonal thermocline and the core of the eddy at about
σθ=26.3 kg m-3 (∼205 to 240 m depth) and takes place during the first half of the lifetime of the eddy.
During this period the variability in the amplitude of the ACE2 is
negligibly small, which might be due to the coarse resolution of the
satellite data. The radius increases up to 50 km with a nearly consistent
rotation velocity at the same time (Fig. 5a, b). During the second half of
the lifetime the radius of the ACE2 slightly increases but the maximum
rotation velocity and therefore the nonlinear parameter decreases (Fig. 5c)
until the decay of the ACE2. Stronger mixing during the first half of the
eddy lifetime could be related to a stronger wind curl near the coast in
comparison to the open ocean (Albert et al., 2010, their Fig. 1b). However,
as the changes might also be due to the fact that the floats might be
located at different positions within the eddy or near the outer edge these
results should be regarded with caution. Often floats are carried with the
eddies near the outer edge of the eddy, avoid the core of eddies, and hence
underestimate the strength of eddies. Especially floats with a parking depth
at 400 m stay near the edge of eddies and shift between cyclonic and
anticyclonic features. If located in the ETSP at southern boundaries of
anticyclones or northern boundaries of cyclones the floats move eastward
while floats at northern boundaries of anticyclones or southern boundaries
of cyclones move westward (see Supplement movie M1).

Eddies play an important role in the weak circulation region of the ETSP.
Regarding the mean flow in the ETSP at the position of the Stratus mooring
the zonal annual mean velocity components for the periods 2011/2012 and
2014/2015 as well as for the period October 2000 to December 2004 are quite
different (between -4.6 and +2.5 cm s-1), while the meridional
components are generally weaker (between -2.1 and 2.3 cm s-1) and more
similar (Supplement Fig. S4). In 2011/2012 the flow component is
south-eastward between 100 and 450 m depth and in 2014/2015 south-westward
in the upper 50 m depth, north-westward at 50 to 250 m depth, and mainly
southward at 250 to 600 m depth. The north-westward flow at 50 to 250 m depth
in 2014/2015 would fit to a mean South Pacific subtropical gyre
circulation (e.g. Ayón et al., 2008); however, the period 2011/2012 showed
mean eastward flow at all depths between 50 and 600 m and in the 2000/2004
period (Colbo and Weller, 2007) the eastward flow component at 45 to 235 m
also showed the opposite flow component. Hence, the different measurement periods
at the Stratus mooring do not support the view of a mean north-westward flow
of the South Pacific subtropical gyre at the mooring location and other
processes dominate the flow field.

Observations from the mooring deployment period March 2014 to April 2015 and
profiling floats deployed near eddies in March 2014 (Fig. 1) show the
importance of eddies in the weak flow region of the ETSP as mesoscale eddies
play a crucial role in water mass distribution. High anomalies of heat,
salt, and oxygen transported within eddies into the ETSP suggests the
influence of seasonal and/or interannual variability in the formation
regions off Peru and Chile.

It is necessary to further gain knowledge about the seasonal, interannual,
and other variability having an impact on the generation of eddies and on
the water mass properties that are trapped within eddies to understand their
impact on the maintenance and shape of the OMZ. The most promising methods
to tackle the open questions would be long-term measurements near the coast,
glider measurements in the centre of the eddies, and accompanying model
investigations.

The used satellite data of SLA can be freely downloaded at
Copernicus Marine and Environment Monitoring Service (CMEMS, 2018,
http://marine.copernicus.eu). The Copernicus Marine
and Environment Monitoring Service (CMEMS) (http://marine.copernicus.eu) has
taken over the whole processing and distribution of the products formerly
distributed by AVISO with no changes in the scientific content.

Microwave OI SST data are produced by Remote Sensing Systems and sponsored by National
Oceanographic Partnership Program (NOPP) and the NASA Earth Science Physical
Oceanography Program. Data are available at
http://www.remss.com/measurements/sea-surface-temperature (last access: 17 August 2017).

Chlorophyll data can be freely downloaded at National Aeronautics
and Space Administration (NASA, 2018, http://oceancolor.gsfc.nasa.gov, 10.5067/AQUA/MODIS/L3M/CHL/2018).

Argo float data and Stratus Ocean Reference Station (ORS) data used in this paper are
available at Pangaea (https://doi.pangaea.de/10.1594/PANGAEA.892463, Czeschel et al., 2018.).

RC, FS, and LS conceived the study. RC
handled the float data. FS collected and interpreted the
satellite data set. RAW is responsible for the Stratus ORS mooring
measurements and the data processing. All authors discussed, wrote, and
modified the manuscript.

The authors declare that they have no conflict of
interest.

Acknowledgements

Financial support was received through Woods Hole Oceanographic Institution
(Robert A. Weller) and the GEOMAR (Rena Czeschel, Lothar Stramma, and Florian
Schütte). The Stratus Ocean Reference Station is supported by the
National Oceanic and Atmospheric Administration (NOAA) Climate Observation
Program (NA09AR4320129, OAA CPO FundRef number 100007298). This work is a
contribution of the Deutsche Forschungsgemeinschaft (DFG) supported project
“Sonderforschungsbereich 754: Climate-Biogeochemistry Interactions in the
Tropical Ocean” (http://www.sfb754.de, last access: 27 June 2018).
Edited by: Matthew
Hecht Reviewed by: two anonymous referees